It is known that a diaphragm cell electrolysis plant is fed with a potassium chloride brine of a certain definite composition. By the application of electricity, the brine contained in the electrolysis cell is subjected to electrolytic decomposition into chlorine and hydrogen with the simultaneous formation of potassium solution, i.e. KOH. An adequate quantity of potassium solution, hereinafter referred to as cell liquor, must continuously be withdrawn from the electrolysis cell, said quantity being related to the production of Cl.sub.2 and H.sub.2. There is generally no direct use for this cell liquor because its KOH concentration is very low while the percentage of residual KCl is appreciable. The cell liquor commonly contains, for example, 100-150 g KOH/l and 180-200 g KCl/l. To convert this cell liquor into a marketable product, it is necessary to raise its concentration to approximately 700 g KOH/l by evaporation which causes the major portion of KCl to be precipitated from the liquor in crystalline form. Evaporation of the liquor and separation of crystalline KCl requires inherently uneconomical multistage facilities of highly corrosion-resistant materials of construction, such as nickel for example. In addition, a quantity of 2 to 3 tons of steam per ton of KOH is needed for cell liquor evaporation, said quantity being dependent on the number of evaporation stages.
The high capital costs of the cell liquor evaporation facilities and the high process energy requirements are bound to burden the final product heavily from the standpoint of sales price.